Integrated circuits continue to be scaled down to smaller dimensions. Interconnects, in particular, are reducing in pitch and volume to accommodate the smaller dimensions. Down sizing the interconnects increases current density in, and resistance of, the interconnects. The increases in current density in process generations since the 0.25 micron process technology exhibit significant reliability issues with respect to interconnects as a result of electromigration and, as a result, current conservative design rules and practices are too conservative, or not sufficiently accurate to keep up with design demands.
Electromigration is a process wherein hydrostatic stress on interconnects, such as a direct current applied to an interconnect in one direction, causes current-induced atomic diffusion due to momentum transfer from flowing electrons to host atoms, or a diffusion of electrons in electric fields set up in the interconnect while the circuit is in operation. The metal at anode-end of the interconnect thins or pulls back from the adjacent via and eventually separates completely, causing an opening or void in the circuit. The metal at the cathode-end of the interconnect forms extrusions and crack insulators which leads to device degradation such as short-circuiting interconnects and diffusing metal into the substrate or, more particularly, into the circuit layer and/or a dielectric layer of the substrate. Electromigration reliability is measured in terms of mean time to failure and activation energy. Mean time to failure (MTF) is typically defined as the number of hours before an interconnect increases in electrical resistance by 30% for certain length and width interconnects at a temperature, such as 250 degrees Celsius with the application of a constant current density such as 1×10^6 amperes per square centimeter.
Activation energy is the energy level at which electromigration begins to occur. Activation energy of pure copper, for instance, is lower than aluminum alloys when used in interconnects but both copper and aluminum are used to form interconnects since copper offers intrinsic advantages over aluminum, including a lower resistivity. Lower resistivity allows higher current densities so a smaller copper interconnect may potentially handle the same current as an aluminum interconnect with the same electromigration reliability. In addition, copper based interconnects, unlike aluminum-based interconnects, are surrounded on at least three sides by refractory metal layers that serve as diffusion barriers. After electromigration leads to voiding in the copper interconnects, current shunts around the voids through the refractory metal layers so that even voids that span the length, width, and thickness of interconnects do not cause open-circuit failure. Copper, however, does not solve the increasing problem of electromigration. The voiding leads to an increase in resistance, and as the resistance increases, the void grows. As a result, problems associated with electromigration increase as the interconnects become smaller and metallization layers become more densely populated with interconnects.
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